Various compact and closed press sections are known in the prior art which include a combination of press rolls, whose press rolls form, as a rule, at least three press nips with each other. Examples of such prior art press sections include, for instance, the "Sym-Press II" press section and "Sym-Press 0" press section (trademarks of the assignee, Valmet Paper Machinery Inc.).
In a "Sym-Press II" press section, the first twin-felt nip is formed between a hollow-faced lower press roll and an upper press-suction roll. The suction roll forms a second nip with the smooth-faced center roll of the press. A third press nip is also formed in connection with the center roll.
In a "Sym-Press 0" press section, the first twin-felt horizontal nip is formed between a lower hollow-faced roll and an upper press-suction roll, after which the web follows the upper fabric as a vertical run into a second nip. The second nip is formed between a hollow-faced press roll separate from the rolls of the first nip and the smooth-faced center roll. In connection with the center roll, a third press nip, which is provided with a felt, is formed in conjunction with a hollow-faced press roll.
In compact press sections, the replacement of press fabrics and of heavy press rolls is problematic. To solve these problems, a "QF press frame" (trademark of the assignee, Valmet Paper Machinery Inc.) has been developed. Different versions of the "QF press frame" are known, for example, from published Finnish Patent Applications Nos. 70,951 and 73,025, as well as U.S. Pat. Nos. 4,608,125 and 4,699,692.
In a "QF press frame", the space placed between the front and rear frame parts and at and above the combination of press rolls is arranged open or openable, so that the upper fabric and at least the upper press rolls can be replaced quickly and easily by making use of the open or openable space.
After its introduction, the "QF frame" soon became widely used and commonly accepted because it provided novel solutions for problems that had existed for a long time.
In addition to the above-described "QF frames", a number of other openable frame constructions of a compact press section are known in the prior art.
First, the "Flip-Top" frame part of the Messrs. Beloit Corporation was introduced on the market, which frame part can be opened in two different stages and which is provided with an articulated joint on top of the combination of rolls, so that an open space cannot be formed directly above the combination of rolls and the press rolls cannot be replaced substantially by direct raising. In this apparatus, the replacement of each the various fabrics requires opening of different parts in the frame part as separate operations.
Various modifications of the "QF frame" have been patented to other parties. Examples of these patents are the U.S. Pat. No. 4,879,002 of Oy Tampella Ab, EP Patent Application 0,348,334 of Beloit Corporation, and U.S. Pat. No. 4,826,572 of Mitsubishi Yukogyo KK.
In these patents, various openable frame parts are suggested, wherein the opening is based either on linear shifting of the intermediate frame parts in the machine direction or on pivoting of said frame parts around a vertical articulated joint.
One problem in the press section of a paper machine concerns vibrations in its frame. These vibrations are formed in the press nips, and they cause variations in the linear loads in the nips. This has a direct detrimental effect on the quality of the paper that is being produced. The vibrations in a press frame are particularly awkward when the vibrations and displacements at the operation side and at the driving side of the frame are of different magnitudes and/or have a phase shift in relation to each other.